1. Field of the Invention
The present invention relates to the field of ice detection and more particularly to a new and improved device for optically detecting the presence and thickness of ice.
2. Discussion of the Prior Art
Ice poses a significant problem in the transportation industry. Throughout the years, numerous accidents have been attributed to ice and ice buildup. With respect to aviation, ice has the ability to compromise airworthiness by changing the flight characteristics and the weight of an airplane, thereby increasing the risk of flying. Often pilots are not aware of the onset of airframe icing until the safety of the flight is in jeopardy. Thus, an icing detection system that alerts pilots to the initial onset of icing is desirable. Furthermore, the optimal use of in-flight de-icing equipment such as aircraft de-icing boots often depends upon the depth of ice buildup. Thus, the need exists for a reliable and affordable means to detect ice and to assess the buildup of ice depth.
Several devices for optical ice detection have been previously disclosed. The first incorporates a sensor at one end of an optical channel comprised of a fiber optic bundle. The fiber optic bundle totally reflects light internally when no ice is present, but when water or ice is present, some of the incident light is refracted externally. Another device attempts to detect ice and ice depth by using two or more optical fibers to transmit light and receive reflected light. The device relies on reflections from the surface at the ice/air interface to assess the presence and depth of ice. Other devices use single optical fibers to detect the presence of ice through the scattering or reflection of light at the end of the optical fiber. These devices suffer from serious technical limitations due to roughness of the ice surface and imperfections in the ice scattering the light.
The present invention is an optical ice detection probe capable of detecting the presence, thickness and continued buildup of ice. The invention works by attaching the probe to the external surface of an airplane such as the wing, fuselage or empannage. The probe has a recessed surface, and the shape of the probe permits ice to build up on the recessed surface. Currently, the shape of the probe is cylindrical however other shapes are possible. At least one beam is passed over and parallel to the recessed surface. Beam detectors are used to monitor the beam after the beam has crossed the recessed surface. As ice develops on the recessed surface, ice will obstruct the beam detector, such that the beam will no longer be detectable by the beam detector. For best results, the recessed surface is mounted directly in the airflow, preferably in the laminar flow. The length of the recessed surface is set to optimize the system for the specific application involved. The beams can be of any useful wavelength, however for best results visible, infrared or ultraviolet beams should be used.
If two or more beams are used, then each beam emitter is paired with a beam detector and the corresponding beam detector only detects the beam from that emitter. This can be accomplished by numerous methods. For example, powering each beam emitter alternately and sampling the output of the corresponding detector, electronically modulating the beam to a specific frequency with corresponding detectors designed to respond to the specific frequency or using an optical filter that passes only specific wavelengths. Additionally, placing an optical filter in front of the detector filters out ambient light. The invention also incorporates redundant temperature sensors to measure the temperature of the probe, and a probe heater to melt accumulated ice to assess continued ice accumulation.
In the preferred embodiment of the invention, two or more beams are used, and the beams are all at the same height with respect to the recessed surface, typically 0.02 inches above the recessed surface. The primary purpose for using two or more beams is for contamination rejection. In the event that one beam is interrupted due to contamination, the other beams are still available for ice detection. In the second preferred embodiment, two or more beams will be positioned at sequential distances from the recessed surface. The beams are positioned such that one beam will be closely parallel and above the recessed surface to indicate the onset of icing, while the other beams will be positioned at subsequent distances from the recessed surface to assess the continued buildup of ice. In a third preferred embodiment, two or more beams will be positioned at sequential distances from the recessed surface and at each sequential position, two or more beams will be used for contamination rejection. It is anticipated that this device will continuously measure the temperature of the probe.
If the temperature of the probe is greater than 10 degrees Celsius and at least one beam is interrupted for greater than 60 seconds then the Probe Inspect LED on the cockpit display unit is activated indicating the possibility of probe contamination. If the temperature is below 10 degrees Celsius, and only one beam is interrupted for greater than 60 seconds, then the Probe Inspect LED on the cockpit display unit is activated indicating the possibility of contamination. If the temperature is below 10 degrees Celsius, and more than one beam is interrupted for greater than 10 seconds, the Ice Warning LED is activated and the Probe Heater is activated. The probe heater will continue to heat the probe until 10 seconds after at least one beam detector is cleared. Then the device is reset to continue monitoring for ice. By counting the number of probe heating cycles the device can provide an assessment of total ice depth, and by measuring the frequency of heating cycles the device can provide a rate of total ice depth formation. With respect to the second and the third preferred embodiment, the probe heater will not activate until the beam the furthest from the recessed surface is blocked. The probe heater will then melt the ice until at least one beam is clear and the device is then reset to continue monitoring for ice.
This device distinguishes between ice and fluid by optical intensity fluctuations and attenuation due to thickness. The presence of fluid on the recessed surface will cause the beam strength and therefore the detector output to fluctuate rapidly. The electromagnetic wavelength of the beams is selected such that a thin layer of ice, water or de-icing/anti-icing fluid on the walls of the recess surface will not produce sufficient optical attenuation to trigger ice or contamination indications. The beam wavelength is such that most of the recessed surface must be filled by ice to trigger warning indications. This typically requires electromagnetic wavelengths above one micron.
It is therefore an object of the invention to provide a simple optical detection system that can detect the presence and thickness of ice. It is another object of the invention to detect the presence and thickness of ice irrespective of the roughness of the formed ice or imperfections in the ice scattering the light. It is an object of this invention to distinguish between fluid and ice. Finally, it is an object of the invention to determine when ice thickness has reached a critical depth.
These and other objects of the present invention will become apparent to those familiar with optical ice detection, and more particularly defined by the appended claims. It being understood that changes in the precise embodiment to the disclosed invention are meant to be included as coming within the scope of the claims, except as insofar as they may be precluded by the prior art.